BRUSSELS (AP) — The Northern Ireland party that props up the government of British Prime Minister Theresa May insists that European Union proposals to avoid a hard border in Ireland after Brexit are “not acceptable.”

Democratic Unionist Party leader Arlene Foster said after a meeting with the EU’s chief Brexit negotiator, Michel Barnier, that there can be no division between Northern Ireland and the rest of the U.K. to accommodate for a soft border with the EU.

The border on the island will be the only land border after the U.K. breaks away as of March 2019 and maintaining open trade and passage is proving to be one of the more intractable issues during the Brexit talks.

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And the DUP fears that as the EU seeks to keep the Irish border as transparent as possible, it is at the same time erecting obstacles between Northern Ireland and Britain.

“What we’re concerned about is to have a barrier between ourselves and the rest of the United Kingdom. That is the critical point for us,” Foster said, adding that 72 percent of Northern Ireland goods go to the rest of the U.K.

Just concluded a constructive meeting with Michel Barnier. There are sensible solutions to the border question. Greater flexibility needs to be shown by Brussels. Unacceptable for NI to be treated separately from rest of UK as set out in the draft EU legal text. pic.twitter.com/LAjg8tRX4T

“The single market that matters to the people of Northern Ireland is the U.K. single market,” she said. “It would be an economic catastrophe if we allowed a border between ourselves and the rest of the U.K.”

Barnier assured her that all constitutional rules of the U.K. would be respected.

“The EU is looking for practical solutions to avoiding a hard border, in full respect of the constitutional status” of Northern Ireland, he tweeted.